ELECTRON-TRANSFER IN PORPHYRIN QUINONE CYCLOPHANES STUDIED ON THE PICO-SECOND AND FEMTOSECOND TIME SCALE

Electron transfer in porphyrin-quinone cyclophanes is investigated by fluorescence and absorption spectroscopy with pico- and femto-second pulses. In nonpolar solvents, the S1 state of the porphyrin shows a lifetime from 300 ps up to several nanoseconds, depending upon the number of quinones and upon their electron affinity. Comparative measurements in polar solvents demonstrate very fast electron transfer on a time scale between 1 and 10 ps. The results are analyzed with the aid of quantum-chemical calculations which give the energy of the charge transfer states and the relevant coupling strengths. For nonpolar solvents, theory suggests fluctuation-induced charge separation and/or direct radiationless internal conversion from the porphyrin S1 to the ground state. In polar solution, the molecules exist in a tilted configuration with strong electronic coupling and charge transfer states well below the S1 level, resulting in fast electron transfer and subsequent charge recombination within 10-40 ps.